在Xcode 6.2中，这也是可行的

let appDelegate = UIApplication.sharedApplication().delegate! as AppDelegate

let aVariable = appDelegate.someVariable

下面是一个UIApplicationDelegate的扩展，它避免了硬编码AppDelegate类名:

extension UIApplicationDelegate {

    static var shared: Self {    
        return UIApplication.shared.delegate! as! Self
    }
}

// use like this:
let appDelegate = MyAppDelegate.shared // will be of type MyAppDelegate

Swift < 3

在AppDelegate类中为ex创建一个方法

func sharedInstance() -> AppDelegate{
        return UIApplication.sharedApplication().delegate as! AppDelegate
    }

叫它"前任"吧

let appDelegate : AppDelegate = AppDelegate().sharedInstance()

Swift >= 3.0

func sharedInstance() -> AppDelegate{
    return UIApplication.shared.delegate as! AppDelegate
}

这和Objective-C中的基本相同

let del = UIApplication.sharedApplication().delegate

extension AppDelegate {

    // MARK: - App Delegate Ref
    class func delegate() -> AppDelegate {
        return UIApplication.shared.delegate as! AppDelegate
    }
}

我在Swift 2.3中使用这个功能。

1. 在AppDelegate类中

static let sharedInstance: AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate

2. 调用AppDelegate

let appDelegate = AppDelegate.sharedInstance